I have the same problem, sometimes i can play for hours without ctd, but sometimes it takes only few min for it to crash. I'm not sure why is this happening. I had dawnguard installed for a long time and i never had this problem before!It could be some mod i installed few days ago idk :/ I read somewhere that if you just del you skyrim ini and skyrimpref problem would be gone, still need to try that out.